1. Field of the Invention
The present invention relates to a display device displaying images by a light-emitting element provided to each pixel, a drive method for the display device, and an electronic unit provided with such a display device.
2. Description of the Related Art
In the field of display devices displaying an image, recently developed for market is a display device (organic Electro Luminescence (EL) display device) using a light-emitting element, e.g., organic EL element, being a current-driven optical element that changes in light emission luminance in accordance with a value of current flowing therethrough.
The organic EL element is a self-emitting element unlike a liquid crystal element and others. The organic EL display device thus requires no light source, i.e., backlight, thereby favorably offering a higher image visibility, a lower power consumption, and a faster element response speed compared with a liquid crystal display device requiring a light source.
The organic EL display device is classified into the passive- and active-matrix types in terms of driving similarly to a liquid crystal display device. The display device of the passive-matrix type is simple and easy to configure, but has a problem of difficulty in being large in size and offering a high definition, for example. Due to such a problem and others, the display device of the active-matrix type has been under active development. In an organic EL display device of the active-matrix type as such, an active element is in charge of controlling a current flowing through an organic EL element provided to each pixel. The active element is generally a TFT (Thin Film Transistor) provided in a drive circuit provided to each of the organic EL elements.
The organic EL elements each have the characteristics of current-voltage (I-V), which are generally known to deteriorate over time (suffer from over-time deterioration). In a pixel circuit in which the organic EL elements are driven by a current, when the I-V characteristics of the organic EL elements are deteriorated over time, a current flowing through a drive transistor shows a change in value. This changes also the value of a current flowing through the organic EL elements themselves, thereby changing also the level of light emission luminance.
In the drive transistor, the threshold voltage Vth and the mobility μ thereof may change over time or fluctuate depending on the pixel circuit due to variations of the manufacturing process. When the threshold voltage Vth and/or the mobility μ of the drive transistor fluctuate depending on the pixel circuit as such, a current flowing to the drive transistor may vary in value depending on the pixel circuit. Therefore, even if any specific one level of voltage is applied to the gate of the drive transistor, the organic EL elements vary in light emission luminance, whereby resulting in the loss of uniformity of the screen.
In consideration thereof, there has been a proposal to maintain uniform the light emission luminance of the organic EL elements with no influence even if the I-V characteristics thereof show some change over time, or even if, in the drive transistor, the threshold voltage Vth and/or the mobility μ show some change over time or fluctuate depending on the pixel circuit. To be specific, proposed is a display device provided with functions of compensation and correction. The function of compensation is for compensating for any variation observed in the I-V characteristics of the organic EL elements, and the function of correction is for correcting any fluctuation occurred to the threshold voltage Vth and/or the mobility μ of the drive transistor. An example includes Japanese Unexamined Patent Publication No. 2008-33193.